1. Field of Invention
The present invention relates to a roll paper printer that prints on a paper web delivered from a roll of paper held freely rotatably inside a roll paper compartment, and relates more particularly to technology for preventing problems caused by the leading end portion of the roll paper unwinding in the transportation direction inside the roll paper compartment and then shifting sideways.
2. Description of Related Art
Roll paper printers print to a paper web that is pulled from a roll of paper held freely rotatably inside a roll paper compartment. Due to the inertia of the paper roll when the paper is pulled from the roll, the portion of paper that is wound at the outside circumference of the roll may unwind and be left in a slack loop inside the roll paper compartment.
When the side walls of the roll paper compartment are simple flat guide surfaces (roll paper guides) located a specific distance apart, the unwound portion at the outside circumference side of the paper roll may shift widthwise and become angled to the normal transportation direction. Even when the side walls of the roll paper compartment are rendered using a plurality of guide surfaces instead of by simple flat guide surfaces, the slack unwound portion at the outside circumference of the paper roll may shift widthwise and become angled to the normal-transportation direction. This can cause such problems as the edges of the roll paper catching on the edge of the guide surface or the edges being folded over as the paper is delivered from the roll paper compartment.
To avoid such problems when the sides of the roll paper compartment are defined by a plurality of roll paper guides, the distance between the left and right roll paper guides disposed along the direction in which the roll paper rotates (that is, disposed substantially parallel to the direction in which the roll paper is conveyed) gradually narrows from the upstream side to the downstream side in the direction of roll paper rotation so that the gap between the pair of roll paper guides located at the most downstream end near the printing position is narrowest. Narrowing the gap between the pair of roll paper guides along the transportation direction of the roll paper thus gradually limits the widthwise movement of the roll paper as the paper is conveyed downstream.
However, because the widthwise movement of the roll paper is only loosely limited in the area where the gap between the roll paper guides is wide on the upstream end of the transportation path, the paper can still shift sideways if the outside circumference part of the roll paper unwinds and goes slack in this area. Furthermore, once the paper shifts widthwise in this area, the paper will be fed biased to the transportation direction from the pair of upstream roll paper guides where the gap is wide to the pair of downstream paper guides where the gap is narrow.
Because a plurality of roll paper guide pairs are disposed between the roll paper guide pair with the widest gap and the roll paper guide pair with the narrowest gap, plural steps protrude to the inside widthwise to the roll paper compartment when seen from the downstream side of the paper transportation direction. As a result, if the paper is advanced biased to the transportation direction, the edge part of the paper that is pushed to one side widthwise to the transportation direction may catch on these steps. The edge part of the paper that is thus caught by these steps is easily folded over and may be conveyed downstream with a crease along the edge. Such creasing occurs particularly easily with thin paper that is not stiff.
One common method of controlling the widthwise position of the paper delivered from the roll is therefore to use a single-sided guide, that is, a guide surface along one lengthwise edge of the paper. If this single-sided guide is rendered using a plurality of roll paper guides, the roll paper guides will be disposed in steps along the paper transportation direction. As a result, if the paper becomes biased to the transportation direction, the edge of the paper will still catch on the steps and be folded over.
Japanese Unexamined Patent Appl. Pub. JP-A-H08-133540 and Japanese Unexamined Patent Appl. Pub. JP-A-H09-255196 teach arrangements for preventing the paper becoming biased to the transportation direction as the paper is delivered from the paper roll.
More specifically, Japanese Unexamined Patent Appl. Pub. JP-A-H08-133540 teaches using a flat guide surface rendered by a single-member paper guide in order to regulate widthwise movement of roll paper or other print medium supplied in a roll. Japanese Unexamined Patent Appl. Pub. JP-A-H09-255196 teaches using a substantially U-shaped frame (having a pair of side guide members and a bottom contact member) that is placed from above onto the paper roll and can rock freely vertically to prevent the portion of the roll paper that is pulled off the roll from shifting sideways.
A problem with the arrangement described in Japanese Unexamined Patent Appl. Pub. JP-A-H08-133540 is that using a paper guide on only one side cannot prevent the paper from becoming biased to the transportation direction.
The arrangement taught in Japanese Unexamined Patent Appl. Pub. JP-A-H09-255196 uses a substantially U-shaped frame. In a compact roll paper printer, however, providing enough space to locate a frame that is large enough to place over the roll from above is difficult. In addition, placing such a frame over the roll increases friction and resistance to pulling the paper from the paper roll. Resistance to roll paper transportation also varies as the diameter of the paper roll increases. As a result, the rate at which the paper is delivered from the roll may also vary.